1. Field of the Invention
The present invention relates to a method for etching phosphate ore, comprising:                a digestion of phosphate ore by an aqueous solution of hydrochloric acid, which results in formation of an etching liquor consisting of an aqueous phase, in which calcium phosphate is in solution, and an insoluble solid phase which contains impurities,        a first separation between the insoluble solid phase and the aqueous phase of the etching liquor,        a preliminary neutralization of an aqueous medium containing calcium phosphate in solution to a first pH which is lower than the pH at which a significant part of this calcium phosphate in solution precipitates in the form of calcium monohydrogen phosphate (DCP), with precipitation of impurities,        an isolation of the precipitated impurities from the pre-neutralized aqueous medium,        a subsequent neutralization of said pre-neutralized aqueous medium to a second pH which is greater than the aforementioned first pH, with precipitation of DCP, and        a second separation between the subsequently neutralized aqueous medium, which is an aqueous solution of calcium chloride, and the precipitated DCP.        
2. The Prior Art
For a long time, methods have been known which provide for hydrochloric etching of phosphate ore (see patents U.S. Pat. No. 3304157, GB-1051521, ES-2013211 and SU-A-1470663 for example).
These methods have the drawback that they generally use, for etching purposes, a concentrated HCl solution of up to 20% and even 30% by weight. The ore to be used has to be of good quality, that is to say must have a high P2O5 content, and fine grinding of the ore is usually required, which increases the costs. During etching, a thermal shock is obtained which is due not only to the exothermic nature of the reaction but also to the dissolution energy which is released, and the insoluble materials are therefore difficult to separate since the liquor obtained is viscous and is loaded with organic materials originating from the ore. Given the high temperature and the concentration of the HCl solution, significant corrosion problems arise.
Also known is a hydrochloric etching method in which the ore is subjected to a first (limited) etching by dilute hydrochloric acid (see U.S. Pat. No. 3,988,420). The aim of this method is solely to dissolve within the rock, by means of this dilute acid, a substantial part of the calcium carbonate but the smallest possible amount of phosphate, which gives a solid phase enriched in P2O5 for the rest of the treatment.
Also known is a hydrochloric etching method as indicated in the introduction (see FR-A-2115244). This method consists in treating ores which contain very little phosphate, in a counter-current process, with several successive concentrations of hydrochloric acid. This process requires complex and expensive equipment to eliminate enormous quantities of sand and other insoluble materials which pollute this ore. This results in a loss of P2O5 during the treatment of the insoluble materials, and this loss has to be recuperated as much as possible by washing steps.
When a phosphate ore is digested in dilute HCl, two results determine the feasibility of the etching operation:                the yield of P2O5 in solution in the liquid phase. Any P2O5 which remains in the solid residue represents a loss and reduces the yield. This yield is expressed in % of P2O5 present in the treated ore.        the level of purity of the end product, which increases as the amount of impurities dissolved in the liquid etching phase decreases. Within the meaning of the invention, the impurities are all the components (anions, cations and heavy metals, etc.) which are not water, P2O5 and calcium and chlorine ions. In the rest of the description, two elements will be selected as standards representative of the level of purity of the end product, namely fluorine and iron. In the text which follows, the end product will be calcium monohydrogen phosphate (DCP) in which the planned maximum content will be 0.2% for F and 0.05% for Fe.        
It should be noted that the greater the yield of P2O5 in solution, the lower the degree of purity; this is because when all the P2O5 is dissolved, generally most of the impurities are also dissolved. A compromise must therefore be made between the desired level of purity for the end product and the minimum permitted yield of P2O5 in solution for the method to remain profitable, which is primarily determined by the cost of the raw material.
When the starting ore is of suitable quality, it has already been known to apply a method comprising a digestion of the phosphate ore by an aqueous solution of HCl having a concentration of at most 10% by weight, a separation between the insoluble materials and an aqueous phase containing phosphate ions, chloride ions and calcium ions, and a neutralization of the aqueous phase in order to precipitate DCP (see international patent application no. PCT/BE03/00111, not yet available to the public on the priority date of the present patent application).
This is because if conditions are stipulated whereby the minimum yield of P2O5 dissolved by dilute HCl is to be 75% and the minimum level of purity of the end product is 40%, it is possible to determine, as a function of one variable of the method, the conditions under which the method can be carried out. Such an example is shown on the graph in FIG. 1, for a given phosphate ore. The percentages shown on the ordinate represent the level of purity of the end product (curve shown in solid line) and the yield of P2O5 in solution (curve shown in dashed line), and the graduation on the abscissa is that of a given variable of the method. In the shaded zone, between the two limit points A and B, are the conditions suitable for obtaining the minimum values mentioned above, in accordance with the teaching of the application PCT/BE03/00111.
FIG. 2 shows a similar graph, but for a different phosphate ore. It can be seen that, for this ore, it is not possible to achieve conditions under which both the minimum yield of P2O5 (point A) and the minimum level of purity (point B) can be met.
The object of the present invention is therefore to develop a hydrochloric etching method in which the ratio between the yield of P2O5 in solution and the level of purity of the end product is optimized so as to allow economically profitable etching of the ore, while carrying out this process using modern and simple equipment of justifiable expense.